I can't find the official FAQ/tutorial/post, but here's the situation:

Owing to the way Shapeways figures volume, things aren't considered hollow unless there's some hole joining the inside to the outside. That is, there has to be a way for the support material on the inside to get out. You can't just have a vacant space in the middle that isn't connected to the outside world.

That said, you *are* allowed to make that connecting hole really really small, such that the support material can't actually get out (and in that case you essentially get the support material for free. Don't abuse this.) But that looks like the problem you have here: you have an empty space in the middle, but it isn't connected to the outside, so it is still considered solid.

What's the link to the post or whatever that explains all this? Also, some of the tutorials are out-of-date: they talk about how overlapping solids are double-charged (they aren't anymore), etc.

I don't know CB Model Pro and I can't see your model (it's set to be hidden, I think), but you could try use "Uniform Mesh Resampling" in Meshlab (on the "Remeshing..." submenu of the "Filters" menu) with a negative offset (use the "Planar simplification" option), invert the normals so that they point inwards, and connect this to the exterior with a tunnel somewhere in the model. You can use a boolean difference operation to subtract a cone shape for the tunnel. Boolean operations are available in Blender (not Meshlab, as far as I know), although they aren't terribly reliable, and are most likely to work with low-resolution models. Similar operations may be available in CB Model Pro.

The tunnel-thing is really specific to Shapeways. I am afraid that no tool will do it automatically.
Look at the video in this model. It is a tutorial (for 3DS Max) to explain how to hollow a die (including the tunnel). Note that if the tunnel is too small, Shapeways automatic tools will just ignore it, so check the price to be sure the hollowing has properly been taken into account.

Hi razhoo,
I have been playing around with different ways of hollowing on zbrush. And there is a way. I am not sure if it is the most efficient way. When i am done I am going to post a zbrush tutorial in full. But till here is some basic ways I have tried.

I should tell you I am a beginner at zbrush, so there is probably an easier way.

WAY #1 : subtract boolean
1. Draw a cube on the canvas, click Edit button then make it a Polymesh3D.
2. In the Subtool palette, click Append button and select the Cylinder. (will automatically be made a Polymesh3D)
3. Click on the Cylinder in the Subtool palette to make it the active subtool.
4. Go to Deformations submenu and, using the XYZ toggles with the Size slider, size the cylinder to what you want. You can also do the same to move it using the Offset slider.
5. Select the Boolean Subtract (middle) icon on the Cylinder subtool.
6. Select the Cube subtool then click Remesh All button to get a cube with a hole.

1.The "boolean Subtract (middle)Icon" is the 2nd symbol from left to right. It is one empty circle and one white circle overlapped.

2.When you deforme the appended object (to be deleted from the initial object) stretch part of it to prtrude through the initial object so that when you subtract it, it will make a hole. The hole is for the sand to fall it during the printing.

3. When you deform and are working on the appended object click the transparency button. (on zbrush4 this is on the right at the bottom next to poly f, ghost,solo,xpose,frame, zyz, l.sym, local. floor,zoom..... buttons) you can push an pull the appended shape in and out of the initial object.

4. On stage 5 when you hit 're mesh all'. On the button there are letter x,y,z. This will make the re meshing mirror whatever axis is turned on. if you are sculpting a figure with the x axis mirroring the entire time then it wont matter if the x is highlighted on the re mesh all button. - but if your sculpture is asymmetrical then you will want all buttons off.

5. On step 5 I have had the best success moving the res slider all the way to the right, and the polish to 2 or 3. I do not now what res stands for but the more to the right the more squares the re mesh uses --- therefor more detail is saved. I polish so low to not loos detail as well.

below steps i use on all my hollowing processes______________
6. then I export it using the 3d printer exporter plugin. ( you might have to use the decimation master plugin as well). export as a stl binary file.

7. then i use netfabb to check the wall thickness. and adjust the size. and that will give me the file to upload to shapeways

Problems:

1. depending on how complex the model: I have found fishers running through the shell of my model. (fisher : my made up word for a mesh tube that connects one side of the model wall to the other forming what seems to be a tube running through the inside) I do not know if shapeways will print that because at that point in your mesh the thickness is 0, or very close to it. I don't know how to fix them.

2. After stage 5. depending on how complex the model: You will still need to use control shift to slice your model open and look at cross sections of your models walls. You will have to subtle use the move and smooth key to make sure all your walls are not close, and not to far from the exterior wall.

3. This is a problem for all my hollowing method. You have to re mesh all (i am talking about a simple re mesh here, just hitting re mesh all) once or twice during your sculpting to make sure that your model has one smooth wall around. i.e. no crumpled areas or walls pushed behind each other.

Conclusion:

1. As of now, i have not found a way to get around the 'fishers' problem. so unless your model is fairly simple this method will probably create these fishers.

2. When you are deforming the appended part you will not be able to do this perfectly to get a 1mm wall all the way around. So- this be time consuming and cost you some more money to print.

1. I litterly mask a circle where i want my hole to be in the model. I then invert the mask so the circle area is now unmasked.

2. then I geometry divide,divide...divide. so that that small circle has a bunch more points.

3. open up sub tool menu, and hit append, ----append the same sculpture you are working on. ( in your subtools you should have 2 of the same sculpture) -- Then click transparency. ( The appended subtool should be transparent now.

4. I use control,shift , drag then alt to hide as much of the masked initial subtool as possible. ( My first subtool should be just the circle, and the second subtool should be a transparent full sculpture) now you are ready to start sculpting the inside walls.

5. Use move and smooth to turn the flat circle into an inner shell the best you can. Make sure the none of the inner wall is poking out of the transparency.

6. shift, control - tap the outside area to bring the entire initial sculpture into view. and hide or delete the appended sculpture.

7. use shift control to view cross sections of your model. The outside wall will be masked so it wont move. This way you can move the inside wall as close/or away form the outside wall. ( i slice the model many times and just wok on the outside edge of the cross section) --- this can take hours.

problems
1. very, very time consuming
2. the shell will not be perfect so will cost you $$$ in printing.

conclusion
1. If you are doing multiple prints the time put into this process is acceptable.

2. IF you just want to hollow a large chunk of the center area but are not to concerned with getting the walls to 1mm accuracy this will be fairly fast and will work.

3. The simpler the model the easier and more efficient this is.

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Way 3 extract
1. open sub tool menu.
2. hit extract button (i set E smt and s smt to zero) (depending on the complexity of your model i set the thickness form .2 to 1. The bigger you want to print the small the extract thickness can be.
3. I will then use control shift to slice my model and correct areas that overlap. (If you have a clean model. meaning your re meshed a couple of time while you were sculpting and have no overlapping areas, this part will not take long. i just look for areas that crease in my model when i am fixing stuff. )

Problems
1. Extracting puffs out the model. So if you are doing portraits this will screw up allot of your detail. The bigger the extracted thickness the more distortion. I do not know how to extract so the distorted wall goes inward.
what i do is make the extracting process part of my sculpting process. What i mean is. i extract before i am finished sculpting. I extract when my model is still a bit general looking. Then after the extraction i sculpt (only add) on to the surface wall. You cant really subtractive sculpt, as you will crush your wall in.

conclusion.
I can get my cheapest most accurate wall this way. But obviously my sculpting process is changed and my freedom to change things toward the end is reduced quite a bit.

This 'extract' process is only good for models of 40x40x40mm or larger really. Once you start going smaller you will have to extract a thicker wall and this will distort the model to much. above 40mm and the extract distortion is manageable. because you can stay around .3 -.5 extract thickness.

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Way #4 re-topography
There is a way to re mesh using retopography. And you can set your wall thickness using this. and then you project the details onto the new topography using the project brush.
I have not figured out re topography yet. and this does not seem very fast at all to me either.

After seeing so many of these kind of threads i thought ok i'm willing to help out (when time permits) and hollow any complex/organic/zbrush models with an exact wall thickness you choose (for ex: 1.5mm for plastics, 3mm for full color sandstone/steel/glass etc.) throughout the inner model's bumps and curves and with NO change to the outer topology.... up to the 500k poly limit or your own limit - for a maximum savings on printing cost - see examples on my own models using the 'view in 3d' java viewer .

Your walls are really great. Your services would save me a tone of time. I would eventually like to learn how to do this on my own. But until i learn how to do this your services are greatly appreciated.
shane

Seems like these kind of features are the holy grail for 3d modelers...Ive been trying to just do it by eye and scaling in sub'd programs with little success, Trying to offset the surface inwards when you might have parts of it that don't want hollowing, or getting your outer details tangled on the inside are all issues too...I'm guessing ill have to give in and use yet another 3rd part app to do this post-initial model at some stage :/